An LTE network is a data packet only network. To support a voice call that does not use Voice over LTE (VoLTE), the LTE specification defines a circuit switched fallback (CSFB) mode in which a user equipment (UE) switches from an LTE mode to a legacy mode to originate or to receive a mobile originated (MO) or mobile terminated (MT) voice call and then switches back to the LTE network after the voice call is released. As an example, a legacy mode can include communication in accordance with a UMTS/CDMA/GSM (universal mobile telecommunications system/code division multiple access/global system for mobile communications) wireless communication protocol. Switching back to an LTE network is defined to follow a normal legacy network's (e.g., for a UMTS/CDMA/GSM network) call release procedure (e.g., perform a cell search after connection release). In this procedure, the LTE network is searched first for a suitable cell. If a suitable LTE cell is found, then the UE camps on the suitable LTE cell. Otherwise, the UE camps on a cell of a legacy network (e.g., UMTS/CDMA/GSM), and will reselect to an LTE cell when an LTE cell is found later using cell a re-selection procedure.
In general, cell selection after a voice call release should be as fast as possible, so that the UE can re-camp on a serving cell and start to listen to paging messages from the serving cell. Otherwise, a next paging message can be missed. Fast re-camp can occur, when the UE selects a cell that uses the same RAT (Radio Access Technology) as the just released voice call. This can most likely be the active cell on which the voice call occurred, or on one of a set of stronger neighbor cells that use the same RAT. With CSFB, after the voice call is released, the UE needs to search for cells of the LTE network first, and then for cells of the legacy network (e.g., UMTS/CDMA/GSM) on which the voice call occurred. This procedure can increase the re-camp time dramatically when the legacy network (NW) does not provide LTE cell information (or provides insufficient LTE cell information) in a RRC (Radio Resource Control) connection release message triggered by the voice call release. Most present network deployments do not provide LTE cell information in the RRC connection release message that triggers the voice call release, as UMTS/CDMA/GSM are established wireless networks.
A long re-camp time after release of a CSFB voice call can result in a poor user experience, as a user can expect to resume LTE service instantly after a voice call is released. Therefore, what is desired is a method of fast return to LTE from CSFB.